The aniline black is a black dye or pigment obtained by oxidation polycondensation of an aromatic amine such as aniline, toluidine and phenylene diamine, and can exhibit a bluish black color which cannot be given by carbon black. The aniline black has been used in various applications including coating materials, printing inks, paints, poster colors, plastics, thermal transfer inks or the like because of a jet-black color inherent thereto. An optimum method for production of the aniline black using an oxidizing agent is the method using a bichromate as the oxidizing agent.
However, the bichromate comprises chromium ions as a pollutant extremely harmful to human body (currently, it is recognized that chromium contained in the aniline black which is coordinated to a dyestuff is safe and harmless because it comprises no toxic hexavalent chromium), and copper ions derived from a copper salt used as a catalyst are also a harmful substance. Therefore, there is an increasing demand for aniline black which can be produced without using any of these harmful substances and is free from such a risk that these harmful substances are included therein.
Conventionally, as to the aniline black comprising no harmful substances such as chromium and copper, there are known the method of producing aniline black which is characterized by oxidizing aniline in the form of an acid aqueous solution thereof with a persulfate (Patent Document 1), and the method of producing aniline black by oxidizing aniline with an OH radical as an oxidizing agent which is generated using hydrogen peroxide and a metal or a metal salt capable of acting as a catalyst for decomposition of the hydrogen peroxide (Patent Documents 2 and 3).
In addition, as a n electron conjugated polymer exhibiting a high blackness and having a highly controlled particle diameter, there is known a black pigment obtained by subjecting a water-soluble high-molecular weight compound, a transition metal compound and a protonic acid to oxidation polymerization using an oxidizing agent(Patent Document 4).
On the other hand, with respect to a polyaniline well known as a conductive polymer, there have been recently proposed a water-soluble sulfonated polyaniline capable of exhibiting a conductivity without addition of a doping agent thereto, and a method for synthesis of the polyaniline. For example, there are known a method of sulfonating a polyaniline with fuming sulfuric acid (Non-Patent Document 1), a method of sulfonating a copolymerized product obtained from aniline and aminobenzenesulfonic acid or a derivative thereof (Patent Document 5), or the like.
In addition, as a black colorant for a non-magnetic developer for electrophotography and a black matrix for liquid crystal displays, there have been used carbon blacks most of which are inexpensive and exhibit an excellent tinting power. In recent years, it has been demanded to attain a high quality of electrophotographic images and a high light-shielding rate of the black matrix. For this reason, studies have been made on increase in concentration of the colorant in the developer or black matrix by increasing an amount of the colorant added thereto. However, the carbon black tends to be hardly dispersed in a binder resin and the obtained resin composition tends to exhibit a low volume resistivity value. As a result, in the case of the former developer, there tends to occur such a problem that the obtained developer tends to be deteriorated in charging performance (deteriorated in charge retention capability). Also, in the case of the latter black matrix, there tends to occur such a problem that since it is difficult to impart a good dispersion stability to a dispersion of the carbon black, the dispersion tends to have a poor fluidity and therefore the resulting black matrix as a resist thin film tends to exhibit a low volume resistivity value.
In general, it is known that the aniline black has a high volume resistivity value as compared to inorganic black pigments such as carbon black. Therefore, to solve the above conventional problems, the use of the aniline black in the above applications has been proposed. With respect to the black colorant for a non-magnetic developer for electrophotography, there has been proposed the use of aniline black as a developer capable of exhibiting a less leakage of charge even under high-temperature and high-humidity conditions and having an excellent charge retention performance (Patent Documents 6 and 7).
Further, there is also known a colorant used in a black matrix for liquid crystal displays which is obtained by coating carbon black with a resin to impart a good dispersion stability and a good fluidity to the carbon black when used in the form of a dispersion thereof (Patent Document 8).